The present invention relates to a wavelength division multiplexing (WDM) apparatus that combines a plurality of optical signals of different wavelengths and optically amplifies the combined signal. WDM apparatuses are classified into two types: a non-transponder type which takes as inputs a plurality of narrowband optical signals of different wavelengths, combines them together, and optically amplifies the combined signal; and a transponder type which has at its front end a plurality of transponders that respectively convert a plurality of wideband optical signals of the same wavelength into a plurality of narrowband optical signals of different wavelengths. The present invention concerns both types of WDM apparatus.
A prior art non-transponder-type WDM apparatus comprises a plurality of variable attenuators which respectively attenuate a plurality of optical signals of different wavelengths with variable attenuation ratios, an optical combiner which combines the optical outputs of the attenuators, and an optical amplifier which optically amplifies the optical output of the optical combiner. The spectrum of the output light is measured with a spectrum analyzer unit and, based on the result of the measurement, each individual variable attenuator is controlled to suppress a variation (tilt) in the output level of each wavelength. If a wavelength deviation greater than an allowable level is detected from the result of the spectrum measurement, the amount of attenuation for that wavelength is set to a maximum to prevent it from affecting its adjacent wavelengths. In a transponder-type WDM apparatus, the plurality of variable attenuators are preceded by a plurality of transponders that respectively convert a plurality of optical signals of the same wavelength into a plurality of optical signals of different wavelengths. The prior art non-transponder-type WDM apparatus and transponder-type WDM apparatus will be described in detail, later, with reference to the drawings.
When there is an unused wavelength in the plurality of wavelengths, that is, when unmodulated light is input, such an unused wavelength must be attenuated sufficiently (to about 50 dB) to prevent it from affecting the optical amplification of the other wavelengths in use. However, in the above prior art WDM apparatus, since the amount of attenuation that each optical attenuator can provide is about 20 dB at maximum, light leaking from the unused wavelength is also amplified together with the optical signals in use. The resulting problem is that the optical power of the optical signals, in use, may degrade, leading to a main signal error. This problem becomes particularly pronounced when the number of wavelengths in use is small.
A second problem with the prior art WDM apparatus concerns the accuracy of each wavelength in the output light. As earlier noted, in the prior art, wavelength deviation of each wavelength is monitored with a spectrum analyzer, with provisions made so that if a wavelength deviation greater than an allowable level is detected for any wavelength, the amount of attenuation for that wavelength is set to a maximum to prevent it from affecting its adjacent wavelengths. However, the spectrum analyzer has the problem that its wavelength resolution is poor and measurements cannot be made with high accuracy. Another problem is that since the wavelength is swept, it takes several tens of seconds from the time a wavelength deviation occurs until the time it is detected, this giving rise to the possibility that an error may be caused in adjacent wavelengths during that time.
Accordingly, a first object of the present invention is to provide a WDM apparatus that is not affected by light leaking from an unused wavelength.
A second object of the present invention is to provide a WDM apparatus that can take corrective action by quickly and accurately detecting a wavelength deviation.
The first object is achieved by the wavelength division multiplexing apparatus of the invention comprising: a plurality of variable attenuators to which a plurality of optical signals of different wavelengths are respectively input, and which respectively attenuate the input optical signals with variable amounts of attenuation; an optical combiner which combines optical outputs of the plurality of variable attenuators; an optical amplifier which optically amplifies an optical output of the optical combiner; a spectrum analyzer unit which measures the spectrum of an optical output of the optical amplifier, and controls each of the plurality of variable attenuators so as to maintain the optical power level of each wavelength at a predetermined level in accordance with the result of the measurement; and optical shutoff means for shutting off an input of an optical signal of a wavelength not used among the plurality of optical signals.
The second object is achieved by the wavelength division multiplexing apparatus of the invention comprising: a plurality of transponders which respectively convert a plurality of optical signals of the same wavelength into a plurality of optical signals of different wavelengths; a plurality of variable attenuators to which the plurality of optical signals of different wavelengths are respectively input, and which respectively attenuate the input optical signals with variable amounts of attenuation; an optical combiner which combines optical outputs of the plurality of variable attenuators; an optical amplifier which optically amplifies an optical output of the optical combiner; a wavelength monitoring device, provided between the transponders and the variable attenuators, for monitoring each optical signal for wavelength deviation; and a controller which sets the amount of attenuation to a maximum value for the optical attenuator corresponding to the optical signal that has been detected by the wavelength monitoring device as having a wavelength deviation greater than a predetermined value.